The present invention relates generally to a photomask used for the fabrication of high-density integrated circuits such as LSIs and a photomask blank for the fabrication of the same, and more particularly to a halftone phase shift photomask that enables projected images of minute size to be obtained and a halftone phase shift photomask blank for the fabrication of this halftone phase shift photomask.
Semiconductor integrated circuits such as LSIs are fabricated by the repetition of a so-called lithography process using a photomask. Possible applications of such phase shift masks as set forth in JP-A 58-173744, JP-B 62-59296, etc. to the formation of microcircuits in particular have already been under investigation. Among others, the so-called halftone phase shift photomask such as one set forth in U.S. Pat. No. 4,890,309, etc. has attracted attention in view of expedited practical applications. For instance, as disclosed in JP-A""s 05-2259 and 05-127361, some proposals have been made in respect of arrangements and materials improved in yields and reduced in costs for the purpose of practical applications.
Here a typical halftone phase shift photomask is briefly explained with reference to FIGS. 7 and 8. FIGS. 7(a) through 7(d) are illustrative of the principles of halftone phase shift lithography, and FIGS. 8(a) through 8(d) are illustrative of a conventional process. FIGS. 7(a) and 8(a) are each a sectional view of a photomask, FIGS. 7(b) and 8(b) are each illustrative of the amplitude of light on the photomask, FIGS. 7(c) and 8(c) are each illustrative of the amplitude of light on a wafer, and FIGS. 7(d) and 8(d) are each illustrative of the intensity of light on the wafer. Reference numerals 411 and 421 are each a transparent substrate, 422 a 100% light-blocking film, 412 a halftone phase shift film, and 413 and 423 are each incident light. By the term xe2x80x9chalftone phase shift filmxe2x80x9d used herein is intended a film, in a single layer or multilayer form, having functions of substantially inverting the phase of transmitting exposure light with respect to the phase of exposure light passing through the air having the same optical length and attenuating the intensity of the light. According to the conventional process, the 100% light-blocking film 422 formed of chromium or the like is provided on the substrate 421 formed of quartz glass or the like, as shown in FIG. 8(a), thereby achieving a simple arrangement where a light transmitting portion of any desired pattern is formed. The light on the wafer has such a fan-shaped intensity distribution as shown in FIG. 8(d), resulting in poor resolution. With the halftone phase shift lithography, on the other hand, it is possible to achieve improvements in resolution, because the phase of light transmitting through the halftone phase shift mask 412 is substantially inverted with respect to the phase of light transmitting through its opening, so that the intensity of light at pattern boundaries on the wafer is reduced to zero as shown in FIG. 7(d) and, hence, such a fan-shaped intensity distribution is prevented.
Referring to the halftone phase shift photomask in general, the optimum phase difference is 180xc2x0, and the optimum transmittance is in the range of 1to 20% (100% for the opening) as determined depending on the pattern to be transferred, transfer conditions, etc. A halftone phase shift mask must be fabricated in such a way as to meet its optimum phase difference and transmittance. Any deviation from the optimum values leads to changes in the correct exposure, etc., ending up with dimensional accuracy drops, a limited tolerance to focus, etc. Thus, the refractive index, extinction coefficient, thickness accuracy and thickness stability of the single layer or multilayer forming the halftone phase shift film are of vital importance.
To ensure the effect of the halftone phase shift photomask, the phase difference and transmittance thereof at an exposure wavelength are of vital importance. To reduce the influences of multiple scattering, etc. between the photomask and lenses in the photolithography process, the reflectance of the mask at an exposure wavelength should preferably be kept low as is the case with conventional photomasks, and most preferably be 20% or less. Regarding the appearance quality, dimensional accuracy, position accuracy, etc. of the pattern formed on the photomask, too, the photomask is required to have similar various properties to those needed for conventional photomasks. To ensure these properties, the optical properties of the photomask, for instance, its transmittance and reflectance at wavelengths at which various inspection and measuring devices are used in the masking process are also of vital importance. To be more specific, the photomask should have a transmittance and reflectance of 50% or less at wavelengths of 488 nm or less.
In this connection, halftone phase shift photomasks may be fabricated by means of PVD or CVD processes. In most cases, however, they are fabricated by a reactive sputtering process wherein main materials are used in the form of targets. The refractive index, extinction coefficient, thickness accuracy and stability of the film are basically determined by the film-formation process. For etching the phase difference shift film at the step of patterning the: phase shift photomask, dry etching such as reactive ion etching is chiefly used. In the process of fabricating phase shift photomasks, washing is repeatedly carried out so as to improve their degree of cleanliness.
For instance, halftone phase shift films comprising any one of tantalum, an oxide of tantalum, a nitride of tantalum and a combined oxide and nitride of tantalum have been proposed as set forth in JP-A""s 05-257264, 07-134396 and 07-281414.
In recent years, as the pattern to be formed becomes fine, it is required to make short the wavelength of exposure light used for lithography, and KrF excimer lasers of 248 nm wavelength are now put to practical use for patterns finer than the so-called 0.25 xcexcm design rule. With expected further size reductions in view, ArF excimer lasers of 193 nm wavelength are under investigation. For halftone phase shift films used for halftone phase shift photomasks, too, the development of materials capable of achieving the optimum phase difference and transmittance for these wavelengths and having stable refractive indices and extinction coefficients are in great demand.
However, conventional halftone phase shift films comprising any one of tantalum, an oxide of tantalum, a nitride of tantalum and a combined oxide and nitride of tantalum, too, have several problems. For instance, a problem with films comprising tantalum or the oxide of tantalum is that when they have a thickness enough to have a phase difference of 180xc2x0 C. at 248 nm wavelength, their transmittance decreases to 1% or less that deviates the range of 1 to 20% thought of as being the optimum transmittance range.
For instance, the thickness vs. 248 nm wavelength phase difference and transmittance relations of a tantalum nitride film formed on a synthetic guartz substrate by a reactive sputtering process using a pure tantalum target in a nitrogen atmosphere are shown in FIG. 9. As can be seen from FIG. 9, the phase difference increases in proportion to an increasing thickness and, at the same time, the transmittance becomes low.
The tantalum nitride film has a phase difference of 180xc2x0 at a thickness of 113 nm, but fails to have the transmittance needed for a phase shift photomask, because its transmittance is 0.06% at that thickness.
Films comprising the oxide of tantalum or the combined oxide and nitride of tantalum may have a phase difference of 180xc2x0 and a transmittance of 1 to 20% as desired, both at 248 nm wavelength. However, these films cannot be inspected with precision by an inspection device for ensuring the appearance quality of a mask or a measuring device because their transmittance exceeds 50% at an inspection wavelength.
For instance, the thickness vs. phase difference and transmittance relations of a tantalum oxide film formed on a synthetic quartz substrate by a reactive sputtering process using a pure tantalum target in a nitrogen atmosphere are shown in FIG. 10. As shown in FIG. 10, the tantalum oxide film may be used as a phase shift mask because its phase difference and transmittance are 180xc2x0 and 13%, respectively, at 81 nm thickness and 248 nm wavelength. A spectral transmittance curve and a spectral reflectance curve for an 81 nm thick tantalum oxide film are shown in FIGS. 11 and 12, respectively. As can be seen from FIG. 11, a transmittance of 80% or greater is obtained at wavelengths of 300 nm or greater. As can be seen from FIG. 12, a reflectance of 29% is obtained at 248 nm wavelength; it is impossible to obtain any sufficiently low reflectance at the exposure wavelength.
On the other hand, various inspection and measuring devices are used in the masking process for the purpose of ensuring various property requirements for a photomask, for instance, its appearance quality, dimensional accuracy and position accuracy. These devices are now mainly used with light of 365 nm to 633 nm wavelengths to make an inspection and measurement through a contrast of light intensity between a light transmitting portion and a light blocking portion. For precise measurement, the light blocking portion, viz., the halftone phase shift film must have a low transmittance or a transmittance of at least 50% or less in the wavelength range of 365 nm to 488 nm.
From the foregoing, it is found that it is very difficult to inspect and measure the tantalum oxide film with precision to check whether or not it is acceptable as a halftone phase shift photomask, and so it is impossible to ensure the quality needed for a photomask.
When a tantalum oxide film is formed at such a thickness as to ensure a phase difference of 180xc2x0 at 248 nm wavelength by using a specific tantalum to oxygen ratio or adding nitrogen thereto to obtain a composite oxide and nitride film, its transmittance may be controlled to some extent. However, when this film is provided in a single layer film, it is impossible to obtain a transmittance of 1% or greater at 248 nm wavelength and a transmittance of 50% or less at wavelengths of 365 nm to 488 nm.
When phase shift photomasks comprising the oxide of tantalum, the nitride of tantalum and the combined oxide and nitride of tantalum are patterned by dry etching, it is difficult to gain precise phase difference control because a transparent substrate such as a quartz substrate is etched simultaneously with the tantalum film by a fluorine-base gas used at the dry etching step, giving rise to a phase difference.
In view of such states of the prior art as mentioned above, an object of the present invention is to provide a halftone phase shift photomask which is controlled with precision in terms of its transmittance at a wavelength applied to inspection, and measuring equipment, so that its quality can easily be assured even when its phase difference at an exposure wavelength is controlled at 180xc2x0 C. with precision and its transmittance is set at 1 to 20% as desired at that wavelength as well as a halftone phase shift photomask blank for the fabrication of the same.
According to the present invention, the aforesaid object is achievable by the provision of a halftone phase shift photomask comprising on a transparent substrate and a halftone phase shift film containing at least tantalum, oxygen, carbon and nitrogen, characterized in that the halftone phase shift photomask has a multilayer structure comprising at least two or more different layers.
Each layer is formed of a film wherein the composition ratio of tantalum atoms and oxygen atoms is such that at least 100 oxygen atoms are contained per 100 tantalum atoms, a film wherein the composition ratio of tantalum atoms and other atoms is such that 100 or less other atoms are contained per 100 tantalum atoms or a film wherein the composition ratio of tantalum atoms and oxygen and nitrogen atoms is such that 50 to 250 oxygen atoms and 1 to 200 nitrogen atoms are contained per 100 tantalum atoms. Two or more such films are laminated one upon another while the thickness of each film is controlled, so that the photomask can have a phase difference of 180xc2x0 and a transmittance of 1 to 13% as desired, both at 248 nm wavelength and a transmittance of 50% or less at a wavelength of 488 nm or less.
Preferably in this case, the multilayer film comprising two or more difference film should include at least one layer in which 100 or less other atoms are contained per 100 tantalum atoms.
The halftone phase shift photomask of the invention has a structure wherein a film with the composition ratio of tantalum atoms and oxygen atoms being such that at least 100 oxygen atoms are contained per 100 tantalum atoms is laminated on a film with the composition ratio of tantalum atoms and other atoms being such that 100 or less other atoms are contained per 100 tantalum atoms, or a structure wherein a film with the composition ratio of tantalum atoms and oxygen and nitrogen atoms being such that 50 to 250 oxygen atoms and 1 to 200 nitrogen atoms are contained per 100 tantalum atoms is laminated on a film with the composition ratio of tantalum atoms and other atoms being such that 100 or less other atoms are contained per 100 tantalum atoms. Thus, the halftone phase shift photomask can have a phase difference of 180xc2x0 and a transmittance of 1 to 20% as desired, both at 248 nm wavelength, a transmittance of 50% or less at a wavelength of 488 nm or less and a reflectance of 20% or less at 248 nm wavelength.
The halftone phase shift photomask of the invention has a structure wherein a film with the composition ratio of tantalum atoms and other atoms being such that 100 or less other atoms are contained per 100 tantalum atoms is directly laminated on the transparent substrate, with subsequent films laminated successively thereon. The film with the composition ratio of tantalum atoms and other atoms being such that 100 or less other atoms are contained per 100 tantalum atoms can be etched with a chlorinous gas whereas the transparent substrate such as a synthetic quartz substrate is unsusceptible to etching with a chlorine-base etching gas. Accordingly, when the phase shift film is dry etched, the film formed on the transparent substrate such as a synthetic quartz substrate, with the composition ratio of tantalum atoms and other atoms being such that 100 or less other atoms are contained per 100 tantalum atoms, can be etched using a chlorine-base gas. Thus, the phase shift film can be patterned while the transparent substrate such as a synthetic quartz substrate remains unetched, so that no phase difference occurs at that portion, resulting in precise phase difference control.
The halftone phase shift photomask of the invention has a structure wherein an etching stopper layer is formed on the transparent substrate, with halftone phase shift films laminated successively thereon. Accordingly, even when the phase shift film is patterned by dry etching using a chlorine or fluorine-base gas, the transparent substrate such as a synthetic quartz substrate remains unetched, so that no phase difference occurs at that portion, resulting in precise phase difference control.
Preferably in this case, the etching stopper layer should be formed of a material such as hafnium oxide, which has sufficient etching resistance and whose transmittance at 248 nm wavelength does not drop even when provided on the transparent substrate with a thickness enough to function as a stopper, for instance, with a thickness of about 50 nm.
It is here understood that the present invention also encompasses a halftone phase shift photomask blank for the fabrication of the aforesaid halftone phase shift photomask.
The advantages of the present invention are now explained. The present invention provides a halftone phase shift photomask and halftone phase shift photomask blank comprising on a transparent substrate a halftone phase shift film containing at least tantalum, oxygen, carbon and nitrogen, characterized by having a multilayer structure comprising two or more different films. By thickness control of each film, the halftone phase shift photomask is allowed to have a phase difference of 180xc2x0 and a transmittance of 1 to 20% as desired, both at 248 nm wavelength, so that improved resolution can be obtained at a lithography step. Having a transmittance of 50% or less at a wavelength of 488 nm or less, this halftone phase shift photomask can also be inspected with precision by means of inspection, and measuring equipment for insuring its appearance quality.
The halftone phase shift photomask, or the blank therefor, of the invention has a structure wherein a film with the composition ratio of tantalum atoms and oxygen atoms being such that at least 100 oxygen atoms are contained per 100 tantalum atoms is laminated on a film with the composition ratio of tantalum atoms and other atoms being; such that 100 or less other atoms are contained per 100 tantalum atoms, or a structure wherein a film with the composition ratio of tantalum atoms and oxygen and nitrogen atoms being such that 50 to 250 oxygen atoms and 1 to 200 nitrogen atoms are contained per 100 tantalum atoms is laminated on a film with the composition ratio of tantalum atoms and other atoms being such that 100 or less other atoms are contained per 100 tantalum atoms. Thus, the halftone phase shift photomask can have a phase difference of 180xc2x0 and a transmittance of 1 to 20% as desired, both at 248 nm wavelength, a transmittance of 50% or less at a wavelength of 488 nm or less and a reflectance of 20% or less at 248 nm wavelength. For this reason, multiple scattering, etc. can be prevented from occurring between the phase shift photomask and stepper lenses at an exposure wavelength in the lithography step, so that more improved resolution can be obtained.
The halftone phase shift photomask, or the blank therefor, of the present invention has a structure wherein a film with the composition ratio of tantalum atoms and other atoms being such that 100 or less other atoms are contained per 100 tantalum atoms is directly laminated on the transparent substrate, with subsequent films laminated successively thereon, or a structure wherein an etching stopper layer is formed on the transparent substrate, with halftone phase shift films laminated successively thereon. This enables only the halftone film to be selectively etched, so that there is no phase difference error which may otherwise result from etching of the transparent substrate such as a synthetic quartz substrate. It is thus possible to provide a phase shift photomask controlled with high precision in terms of phase difference.